Spatial structure of synchronized inhibition in the olfactory bulb

Olfactory sensory input is detected by receptor neurons in the nose which then send information to the olfactory bulb, the first brain region for processing olfactory information. Within the olfactory bulb, many local circuit interneurons, including axonless granule cells, function to facilitate fine odor discrimination. How interneurons interact with principal cells to affect bulbar processing is not known though the mechanism is likely to be different than in sensory cortical regions since the olfactory bulb lacks an obvious topographical organization; neighboring glomerular columns, representing inputs from different receptor neuron subtypes, typically have different odor tuning. Determining the spatial scale over which interneurons such as granule cells can affect principal cells is a critical step towards understanding how the olfactory bulb operates. We addressed this question by assaying inhibitory synchrony using intracellular recordings from pairs of principal cells with different inter-somatic spacing. We find that in acute rat olfactory bulb slices, inhibitory synchrony is evident in the spontaneous synaptic input in mitral cells separated up to 300 μ m. At all inter-somatic spacing assayed, inhibitory synchrony was dependent on fast Na + channels, suggesting that action potentials in granule cells function to coordinate GABA release at relatively distant dendrodendritic synapses formed throughout the the dendritic arbor. Our results suggest that individual granule cells are able to influence relatively large groups of mitral and tufted cells belonging to clusters of at least 15 glomerular modules, providing a potential mechanism to integrate signals reflecting a wide variety of odorants.